Rat liver phenylalanine hydroxylase has been crystallized. A procedure has been developed for purifying to homogeneity large amounts of cloned rat liver phenylalanine hydroxylase from E.coli extracts. The physical and catalytic properties of the cloned enzyme appear to be identical to the enzyme from rat liver. A procedure has been developed for obtaining pure kidney phenylalanine hydroxylase. Isolated by this procedure, the enzyme retains its unique highly activated state. Phenylalanine in vivo induces phosphorylation-activation of rat liver phenylalanine hydroxylase.